THE MANGOS SHALE 595 



avoid the introduction of new names and to refer to the subdivisions as 

 zones and members, using the names adopted by Ilerrick and Johnson 

 (77) as follows: 



The Gastropod zone occurs near the base of the Mancos in a shale for- 

 mation 35 to 100 feet thick. In the shale are lenses and concretions of 

 earthy limestone which contain great numbers of fossils in the Rio 

 Puerco field. This shale is readily recognized in the other coal fields 

 here described, but the fossils contained in it elsewhere are not so numer- 

 ous as they are on the Rio Puerco. In the Cerrillos field it contains thin 

 beds of coal near the base and in the Rio Puerco field carbonaceous shale. 

 It seems probable that this may be the horizon of some of the so-called 

 Dakota coal of the southwest. 



Above this shale is a series of yellow sandstones about 150 feet thick on 

 the Rio Puerco and thinner in some of the other fields. It thickens west- 

 ward and thins toward the east. Herrick and Johnson called it the Tres 

 Hermanos sandstone, and this name may be used to designate the zone 

 of yellow sandstone that occurs near the base of the Mancos in all of the 

 fields described in central New Mexico west of the mountains. It seems 

 to represent some of the sandstones of Benton age which are coal-bearing 

 in western New Mexico. 



The principal part of the Mancos shale occurs above the Tres Harmanos 

 sandstone. It is a more or less homogeneous shale 1,200 to 2,000 feet 

 thick in the Durango region, about 1,000 feet thick in the Rio Puerco 

 field, and considerably thicker in the fields east of the Rio Grande. It is 

 not divisible lithologically into Benton, Niobrara, and Pierre, but the fos- 

 sils contained in it prove that it contains time equivalents of the Benton, 

 probably the Niobrara, and some of the Pierre. Two zones of fossiliferous 

 concretions have been described within this shale in the Rio Puerco field, 

 but it is not known how definitely they can be recognized in other fields 

 and their value as horizon markers is doubtful. A Concretion (Septaria) 

 zone occurs in the Rio Puerco field (77) in close association with the Tres 

 Hermanos sandstone. This zone was recognized by Shinier and Blodgett 

 (109) in several places between the Rio Puerco field and Cabezon, but 

 its occurrence east of the Rio Grande is doubtful. A Ceplialopod zone 

 (77) occurs in the Rio Puerco field 600 feet or more stratigraphically 

 above the Concretion zone. It is characterized by great numbers of 

 limestone concretions containing cephalopods and other shells. This zone 

 seems to be fairly persistent throughout the Rio Puerco coal field and to 

 be recognizable in many places between this field and Cabezon. East 

 of the Rio Grande a zone of concretions containing the fauna of the 

 Cephalopod zone occurs about 700 feet above the base of the Mancos 



